1. Field of the Invention
The present invention relates to a printed circuit board for preventing electrostatic discharge damage, and more particularly to a printed circuit board achieving this without the need for added materials.
2. General Background
In general, electronic products including printed circuit boards (PCBs) are susceptible to damage by electrostatic discharge during manufacturing, assembly, transportation and utilization. Therefore electrostatic discharge protection is necessary for electronic devices, especially printed circuit boards. The trend toward miniaturization of electronic components on printed circuit boards has lead to a reduction in compression and current surge resistance of electronic components. Therefore electrostatic fields and electrostatic currents from electrostatic discharges are more likely to damage precision electronic components on printed circuit boards. Damage resulting from an electrostatic discharge may impair the functioning of entire electronic devices.
Referring to FIG. 3 and FIG. 4, a conventional four-layer printed circuit board includes a signal layer 11, a power layer 15, a ground layer 17, and a plurality of electronic components 21. The printed circuit board defines a plurality of through holes 31 therein, and each through hole 31 has an electrical connection with the ground layer 17 through an inner wall of the through hole 31. Typically, the through holes 31 have metal coatings on the inner walls thereof. The electronic components 21 on the printed circuit board are connected to the ground layer 17 through pins and solder balls, so that the electronic components 21 can be grounded. The ground layer 17 can also provide EMI (Electro Magnetic Interference) shielding for the printed circuit board.
During the manufacture and assembly of printed circuit boards, electrical charges on the surface of an operator's body may be released through electrostatic discharge on contact with the printed circuit boards. The body of the operator commonly contacts an edge of the printed circuit board; therefore in general, the electrostatic discharge takes place on the edge of the printed circuit board. Because electrostatic current often discharges to the ground through a route which has a shortest distance and minimal impedance, the electrostatic current flows to the through holes 31 from the edge of the printed circuit board, and in doing so passes through one or more of the electronic components 21 on the printed circuit board. The electrostatic current may thereby damage the electronic components 21.
There are many means of preventing damage from electrostatic discharge on a printed circuit board. A common means is to design a protective circuit on the printed circuit board. However, in designing a protective circuit, each component's function and position on the printed circuit board should be considered. Additionally, a protective circuit requires additional electronic components and layout spaces on the printed circuit board. Furthermore, this kind of design is complex and increases the cost of manufacturing.
What is needed is a printed circuit board which can prevent damage due to electrostatic discharge without costly additions.